Golf balls having multi-cover layers are known as is the use of ionomers and highly neutralized polymers as cover layers. Several patents and published applications in the prior art teach that it is essential for cover materials to have good flow properties, in particular they should have a high melt flow rate as demonstrated by having a high Melt Flow Index (MFI). For example, U.S. Pat. No. 7,572,195 to Egashira et al., U.S. Pub. Appl. No. 2010/0167841 to Okabe et al., and U.S. Patent Application Serial No. 2010/0190580 to Higuchi et al. all teach compositions for use as golf ball inner cover layers that have MFI's of greater than 2. Other references such as U.S. Patent Application Serial Nos. 2010/0069173 to Okabe et al. and 2010/0075777 to Shigemitsu et al. teach compositions for use as golf ball inner cover layers that are blends of low MFI materials and high MFI materials. Other references teach a broad range of MFI, such as U.S. Patent Application Serial No. 2011/0092312 to Shigemitsu, but only measured at a relatively low temperature, 190° C., and are silent as to the need for the material to be low melt flow at the higher temperatures at which inner cover leakage may occur.
Generally, golf ball cover layers are formed from thermoplastic and/or thermoset compositions. The present invention, however, provides a novel multi-cover layer golf ball construction wherein an inner cover comprises a very low melt flow thermoplastic, and an outer cover layer comprises a thermoset or thermoplastic composition. A problem with molding a thermosetting composition requiring an elevated temperature and/or pressure to cure, over a higher melt flow thermoplastic composition such as an ionomer, is that the ionomer tends to flow out or “leak” out through the thermosetting layer during overmolding—in general, there exist tremendous difficulties in molding high-temperature thermoset materials over any soft layer. The invention herein seeks to reduce or eliminate “leakage” by either 1) reducing the melt flow of the ionomeric material or 2) otherwise increasing the heat resistance of the ionomer by modifying either the ionomeric resin itself prior to molding or via a post-mold treatment to the ionomeric golf ball layer.
Ionomeric materials have long been used as layers of golf balls, very commonly as inner or outer cover layers. Ionomers have excellent toughness, crack resistance, resilience, and a wide range of hardness values and moduli, which make them ideally suited for these types of layers. Methods have been developed to compression or injection mold ionomers into golf ball layers-such methods involve heating the materials to soften and melt them thereby promoting flow to form the desired layers. Ionomers have relatively low vicat softening points (47-71° C.) and low melting temperatures (70-96° C.) which makes them readily moldable but also gives them inherently low resistance to heat and very poor high-temperature properties. As such, the inventive thermoplastic inner cover layers attempt to make use of materials having very low flow at elevated temperatures and/or high resistance to heat.
Most ionomers suitable for conventional golf ball layers have a percent neutralization of from 19 wt % to 69 wt %. Higher levels of neutralization have previously been unsuitable for use by manufacturers or disclosed in the prior art as being useful, without addition of high levels of metal cation-fatty acid flow modifiers, due to the difficulty of molding more highly-neutralized ionomers (>70 wt % neutralization). Whereas the conventional low-neutralization (19-69 wt %) ionomers may be easily injection molded at temperatures of about 300° F. (149° C.) to 450° (232° C.), the highly-neutralized and/or treated ionomers of the invention must be molded at elevated temperatures of from about 500° F. (260° C.) to 680° F. (360° C.) and, more preferably, about 550° F. (288° C.) to 650° F. (343° C.), temperatures previously thought undesirable. When compression molding conventional ionomers (using a two-step process in which half shells are first injection molded, then placed around a core and compression molded into a ball) temperatures as low as 250° F. (121° C.) and typically from about 250° F. (121° C.) to 350° F. (177°) are used. The inventive ionomers must be processed at temps well above 350° F. (177° C.).
The inventive constructions herein (i.e., thermoset or thermoplastic outer cover layers molded over a thermoplastic ionomeric layer) additionally involve the molding of a material requiring curing or processing at a temperature well above the softening and melting temperature of conventional ionomers. This presents a problem that the invention herein seeks to solve, that is, provide ionomeric compositions that will not significantly soften and flow at the conditions that occur when overmolding with a material that requires elevated temperatures to form said overmolded layer.
Commercially ionomers are available in a wide range of melt flows, the melt flow being determined by the degree of neutralization of the acid moiety of the acid copolymer with various metal cations, optimized for physical properties such as toughness and elongation while maintaining melt-processability. Neutralization to 90% and higher is known but is not considered a commercially-viable and usable product because of the loss of melt-processability (producing a low melt flow or intractable material), particularly for copolymers with high acid levels. For example, U.S. Pat. No. 6,777,472 generally describes a process for modification of highly-neutralized ionomers by the addition of a sufficient amount of specific organic fatty acids (or metal salts thereof) in order to maintain melt-processability—unmodified, highly-neutralized ionomers are typically considered unworkable materials because of their low-melt-flow properties.
Various methods of covalent crosslinking the outermost cover of golf balls are known. For example, U.S. Pat. No. 5,891,973 generally discloses an ionomer-covered golf ball that is irradiated via electron beam exposure to increase the resistance to scuff and cut resistance when impacted with a golf club. Covalent crosslinking of non-ionomeric golf ball cover materials with the addition of peroxide is generally disclosed in U.S. Pat. No. 6,303,704 which is also aimed at improving the scuff and cut resistance of softer covers. Ionomer outermost covers, particularly low modulus ionomers are susceptible to softening when exposed to elevated temperatures, thereby losing dimple definition and negatively impacting aerodynamic properties of the ball. One method to overcome this drawback is the irradiation via electron beam or exposure of the dimpled golf ball to gamma radiation, which is generally disclosed in U.S. Pat. No. 6,350,793. None of these references, however, disclose using novel low-melt-flow (or altered, temperature resistant) thermoplastics as a core layer sandwiched between two, thermosetting rubber core layers.
There remains a need, therefore, for low melt flow and/or high temperature resistant thermoplastic ionomeric materials for use in the novel multi-layer golf ball covers herein. The use of these compositions significantly reduces or eliminates the “leakage” of the ionomeric layer into or through the outer core layer, thereby giving a ball having improved consistency of properties as well as improved durability and much reduced susceptibility to breakage when struck with a club head.